Centrifugal pumps typically comprise an impeller rotatably mounted in a pump body for impelling fluid radially outward from an inlet to an outlet. The impeller is typically connected to a rotating shaft and journalled within pump body bearings to rotate within the pump body. The impeller carries a plurality of radially extending vanes which engage fluid during rotation to centrifugally force fluid radially outward.
Many well known advantages and uses exist for centrifugal pumps. However, it is also well known that centrifugal pumps have had problems with heat production and pump efficiency when input or output parameters are varied. An example of one such problem occurs when the impeller is rotating at a given or fixed input speed to provide for high output flow when only a low output flow is desired. Under such conditions, the pump creates excess centrifugal force not needed for low output flow. The excess centrifugal force has typically caused an undesirable increase in pressure head, viscous losses and counterflow losses. The end result has been an undesirable increase in the fluid temperature and an overall decrease in pump efficiency.
Various approaches aimed at reducing the problems associated with centrifugal pumps are disclosed in several U.S. patents. One prior approach varies the geometry of the vanes carried by the impeller. The pumps of this approach are commonly referred to as variable geometry centrifugal pumps, also sometimes referred to as variable performance, variable flow or variable capacity centrifugal pumps. U.S. Pat. Nos. 4,828,454 to Morris, 4,070,132 to Lynch, 3,482,523 to Morando, 2,927,536 to Rhoades and 2,957,424 to Brundage disclose various forms of a variable geometry pump. As generally disclosed in the aforementioned patents, the typical approach to provide a variable geometry centrifugal pump has been to axially translate a shroud to and from the impeller to vary the axial width the vanes and the flow channels between adjacent vanes. The shroud is typically plate shaped and provides female grooves or slots for receiving the vanes. The shroud may be actively translated about the vanes during pumping operation by various fluid, electrical or mechanical actuators and other forms of actuating devices. Some of the various actuating mechanisms are disclosed in these aforementioned patents.
Grennan, U.S. Pat. No. 3,806,278 also shows a similar approach to the aforementioned variable geometry centrifugal pumps but on a different type of pump impeller for a different type of pump. In Grennan '278, a single vane extends axially in a spiral fashion around a central shaft in auger-like or screw like fashion. To vary the width of the spiral flow channel defined by the vane. Grennan '278 discloses a threaded hub type shroud with a spiral female slot for receiving the vane. The hub screws on to the vane to fill in portions of the flow channel near the shaft and decrease the inner diameter of the vane.
Despite any improvements provided by these attempts at variable geometry centrifugal pumps, these pumps have had several drawbacks. One drawback is that of limited design flexibility. Past approaches typically have relied on reducing the axial width of the vanes and associated flow channels. Such approaches are limited in that the flow channels still must be wide enough to efficiently transmit fluid from the inlet to the outlet. Flow channels which are too narrow have problems such as reduced pump efficiency through increased drag. Another drawback in many previous approaches is that the movable shroud can cause increased pump wear or leakage between the outlet and inlet as the shroud axially translates and rotates in relation to the pump body. Yet another drawback existing with these past attempts is their inability stabilize or control pressure head, maintain pump efficiency, and prevent heat production in a prudent or satisfactory manner for many desired applications.